CHEYENNE, Wyo. (AP) — In its prime, a massive steam locomotive known as Big Boy No. 4014 was a moving eruption of smoke and vapor, a 6,300-horsepower brute dragging heavy freight trains over the mountains of Wyoming and Utah.

It's been silent for half a century, pushed aside by more efficient diesels, but now it's coming back to life. The Union Pacific Railroad is embarking on a yearslong restoration project that will put No. 4014 back to work pulling special excursion trains.

"It's sort of like going and finding the Titanic or something that's just very elusive, nothing that we ever thought would happen," said Jim Wrinn, editor of Trains, a magazine that covers the railroad industry.

"Something that's so large and powerful and magnificent, we didn't think any of them would ever come back," he said.

The American Locomotive Co. in Schenectady, N.Y., built 25 of the legendary monsters to Union Pacific's specifications between 1941 and 1944.

An unknown worker scrawled "Big Boy" on the front of one of the engines when it was under construction.

"It came out one day, and it had 'Big Boy' in chalk on it. And from that day forward, it was Big Boy," said Ed Dickens Jr., Union Pacific's senior manager of heritage operations, who will oversee the restoration at the railroad's steam shop in Cheyenne, Wyo.

Dickens and his crew recreated the chalk inscription on No. 4014 when they began to move it.

The Big Boys were the largest steam locomotives ever to work the rugged terrain of the American West, and by most standards the largest anywhere in the world, said Gordon McCulloh, a meticulous historian of Union Pacific steam power.

Big Boys are 132 feet (40.2 meters) long, including the tender, which carried coal and water. They weigh 1.2 million pounds (540,000 kilograms) with a full load of fuel. They are essentially two engines under one boiler, with two sets of eight drive wheels, each set powered by two enormous cylinders nearly two feet across.

And they aren't just big, McCulloh said. They were engineered to reach 80 mph (130 kph), though the railroad never intended to run them that fast. The point was to fine-tune the locomotives so they stayed in balance at any speed and didn't beat themselves up with their own powerful forces.

"You get all that machinery to live in harmony," McCulloh said.

Seventeen Big Boys were scrapped when they were pulled from service, but eight survived and are on display around the country. Union Pacific chose the 4014 for restoration because it spent more than 50 years in the friendly climate of Southern California, at the RailGiants Train Museum at the Los Angeles County Fairgrounds.

The museum took good care of the locomotive, Dickens said, and many of the original components are usable.

RailGiants agreed to give the Big Boy back to the Union Pacific in exchange for some other equipment to display.

No. 4014 has been moved to a Union Pacific shop in Colton, Calif., where a crew is preparing to tow it across Nevada, Utah and Wyoming to Cheyenne, arriving May 8. The UP maintains two other working steam locomotives there, and Dickens and his crew of seven can manufacture almost anything they need.

"This equipment, nobody's going to come in and fix it when it breaks down," he said. "We're it."

Restoration is expected to take three to five years. The railroad would like to have the Big Boy operating by 2019 for the 150th anniversary of the driving of the Golden Spike in Utah, which linked the Union Pacific with the Central Pacific and completed the first transcontinental railroad.

Overjoyed railroad fans are following closely. Wrinn heard from a U.S. soldier serving overseas who was watching live video of the project. People have offered to donate money or even buy lunch for the Union Pacific steam crew, Dickens said, but he allowed that the Omaha, Neb.-based railroad — No. 138 on the Fortune 500 list — can afford the project on its own.

Dickens declined to say how much restoration will cost.

Pouring money into a 73-year-old mechanical relic makes good marketing sense, said Glen Gilmore, a digital and social media marketing expert who also teaches at Rutgers University.

"Nearly everyone has a fascination with trains," Gilmore said in an email to The Associated Press. "Giving people an image that immediately calls to mind so many good things about an industry and brand is a smart marketing move."

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Follow Dan Elliott at http://twitter.com/DanElliottAP